


rules of engagement

by copacet



Category: Life with Derek
Genre: F/M, Humor, Marriage Proposal, Post-Canon
Language: English
Status: Completed
Published: 2020-08-09
Updated: 2020-08-09
Packaged: 2021-03-05 21:27:57
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,594
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/25502116
Author URL: https://archiveofourown.org/users/copacet/pseuds/copacet
Summary: Three times Derek Venturi proposed to Casey McDonald: twice when she thought he was joking, and the one time she realized he was serious.
Relationships: Casey McDonald/Derek Venturi
Comments: 26
Kudos: 137
Collections: Just Married Exchange 2020





	rules of engagement

**Author's Note:**

  * For [klutzy_girl](https://archiveofourown.org/users/klutzy_girl/gifts).



This was it. The day before the first day of the rest of Casey’s life.

Or, well, maybe the first day of classes would be the first day of the rest of her life. Moving into her dorm room and pre-orientation would be the week before the first day of the rest of Casey’s life, and this was the day before that, which was why it was _totally justified_ that she’d been freaking out since she woke up and had rearranged her clothes inside of her suitcases seven times since dinner.

Of course, just because Casey was well-prepared didn’t mean that everyone would be. She knocked on Derek’s door, then pushed it open without waiting for a response. 

“You’d better be all packed,” she announced. “If you mess around in the morning because you can’t find your hockey stick or something, I’m leaving without you.”

Derek, sitting at his computer, failed to react to her entrance. “Yeah, about that,” he said. “I’ve been kind of thinking about not going.”

“What?” Casey asked blankly. “Not going?”

“Not going to Queens,” he clarified. “Or to university in general, I guess.”

Casey stared at him. “What?”

Derek shrugged, keeping his eyes fixed on his computer screen. “This whole four-more-years-of-school thing just doesn’t seem like it’s for me. Too much studying and all that.”

“But,” Casey said. “But, you have to go.” She could hear her voice getting higher-pitched with each word. “You have to, to take classes and get your degree, and what about your hockey scholarship?” And what about _her?_ She’d thought—they were going to go together, they were going to—

Derek shook his head, still not looking at her. “It’s too late, Casey. I’ve already made alternative arrangements.”

“You _have?_ ” Casey’s stomach twisted, and something tight and painful squeezed her chest. Not that she wanted Derek there with her at university, or anything like that. But she’d spent the whole summer mentally preparing for him to be there, so the idea that he might not came as a surprise, and she’d _never_ liked surprises.

“Yep,” Derek said, leaning back in his chair and lacing his fingers behind his head. “I’ve decided to run away and join the circus.” He met her gaze for the first time since she’d walked in, finally allowing her to see the glint in his eyes.

The weight in her chest lifted in a heady rush of irritation and relief. “Der- _ek_!” She picked an eraser up from his desk and threw it at him; it bounced off his forehead. “Not funny! Your education is no joking matter!”

He grinned up at her. “Who’s joking? I’ll have you know that I think I would make an excellent trapeze artist.” He ducked the pencil she threw at his head, then leaned forward over his desk. “You should come with me, Case,” he told her. “Become, like, a lion tamer or something.”

Casey fixed her face into what she hoped was a picture of sincerity. “I _would_ be better at that than you'd be as a trapeze artist,” she said as seriously as she could manage. Derek straightened his shoulders, visibly perking up at the possibility that she was going to play along. “Unfortunately,” she added, “I’m not all that big on cats.” 

Derek recovered quickly. “Fine,” he said. “So we’ll just rob a bank instead.”

Casey snorted.

“We’ll go on the run,” Derek continued. “Become outlaws, change our names, and flee the country in a daring escape by sea.”

“Well, we’re definitely not doing _that_ ,” Casey said. “And you know why?”

He folded his arms and cocked an eyebrow at her. “Why?”

“Because,” Casey said, in as haughty a tone as she could muster, “that is so cliché. Robbing a bank? Where’s your imagination? If I ever become a criminal, my crimes will be high-class and interesting.”

Derek held up his hands in mock surrender. “Okay, _okay_ , Princess. Hollywood, then,” he said. “We wait tables for a couple years until we get our lucky breaks—me as a director, and you as an actress. And neither of us has to take a math class ever again! Think about it, Case.”

“Nope,” Casey said. “Sorry, D. I’m not doing the waitress thing again, and you know it.”

Derek snapped his fingers in disappointment. “Fine,” he said. “Bigfoot hunters. We should totally become Bigfoot hunters.”

Casey cracked up. “Hunting an endangered species?” she scolded once she had recovered. “How could you, Derek? What would Lizzie think?”

“C’mon, we don’t have to _shoot_ him,” Derek said. “It. Whatever. The point is, we’d just be trying to capture a video. You could be the bait, and I’ll carry the camera.”

Casey shook her head. “That sounds like an invasion of privacy. I’m afraid my morals just won’t allow it.”

“Well,” Derek said lightly, “we could always elope to Las Vegas. You know, find an Elvis impersonator to tie the knot and all that.”

Casey sputtered. “Okay, first of all, we’re stepsiblings and I’m pretty sure that’s illegal, but also, I am _not_ getting married by an Elvis impersonator.”

Derek rolled his eyes. “It is _so_ not illegal,” he told her, in the confident tone of a person explaining something very obvious to someone very stupid. “I’ve looked it up.”

Casey opened her mouth to respond, and then closed it again. He’d looked it up? He’d _looked it up?_

“And it doesn’t have to be Elvis,” Derek added quietly. He was looking directly at her.

“Uh,” said Casey.

The grin made a sudden reappearance back on Derek’s face. “Or we could just join the mafia!” he suggested cheerfully.

“The mafia!” Casey said, so grateful for the change of subject back onto solid ground that for a moment she forgot she was supposed to be shooting him down. “Uh, right, well, you might be Italian, but I’m not. So.”

It wasn’t her best effort, but thankfully Derek didn’t call her on it. “Scuba divers in the Bahamas,” he suggested instead. “We’ll go down there thinking we’re just looking for cool fish and awesome beaches, except we’ll totally end up finding long-lost pirate treasure...”

* * *

Casey woke up with Derek’s arm draped around her, and suppressed the urge to scream into her pillow as she remembered precisely what she’d done the night before, and who she’d done it with.

Scrambling out of bed, she retreated to the bathroom and tried not to hyperventilate. 

She’d had sex with Derek. With her stepbrother. She, Casey McDonald, was now officially the kind of person who had sex with her stepbrother. Not just the kind of person who noticed that he was cute, in a vaguely annoying sort of way, or the kind who felt weirdly jealous when he dated someone else for more than two weeks, but the kind who _had sex_ with him, like one of those girls in those porn videos she’d watched accidentally.

This was _so_ not part of her carefully-laid-out-and-color-coded plan for her third year of university. 

Taking a deep breath, Casey looked at herself in the mirror. “This is fine,” she told her reflection. “I’m an adult. I can handle this.” For several seconds, she felt proud of her own ability to keep it together—until her gaze drifted downward, and she noticed the red marks marring her neck. Casey let out a quiet yelp and fled back into her bedroom.

Derek, snoring facedown on the bed, slept on blissfully.

Casey stared at him, wishing simultaneously that he would wake up, so that she wouldn’t be the only one freaking out, and that he wouldn’t wake up, because the thought that he _might_ freak out at having had sex with his keener stepsister left an uncomfortable tightness in her throat. 

Would Derek make it weird? Would he laugh at her? What if he wanted to do it again? What if he _didn’t_ want to do it again?

Casey marched over to her wardrobe and pulled out some fresh clothes; Derek continued snoring resolutely as she dressed and slipped out the door into the Derek-free zone of the apartment’s small kitchen. The kitchen was otherwise empty; her roommate, thankfully, had told Casey that she’d be celebrating the end of finals by spending the night with her boyfriend.

That left Casey alone to deal with the aftermath of spending the night with _her_ —well, with Derek. Trying not to think about it, she started pulling flour and baking powder out of the cupboard.

Cooking breakfast after a one-night stand was the normal, polite thing to do, wasn’t it? Not that Derek was a normal one-night stand: cooking breakfast while her stepbrother overslept in another room was, if nothing else, annoyingly familiar.

Fingers clenching tightly enough around the whisk that her knuckles turned white, Casey stirred the pancake mix until she’d ground every single lump in the batter out of existence. Pressing her lips together, she stared down at the mixture as if it held the answers to the secrets of the universe—or at least to her love life—then returned to the cupboard. Setting up her kitchen scale, she carefully weighed out six ounces of chocolate chips and added them to the mix.

The process of measuring out the batter into equally-sized dollops on the griddle, as close to perfectly circular as she could make them, was calming, and by the time Casey had built up a small, delicious-smelling stack on a plate on the counter, her racing thoughts had calmed into a single-minded focus on making sure each pancake was precisely the right shade of golden-brown when she flipped it over. 

When the door to the bedroom opened, she dropped the spatula. After catching it with what she thought was a graceful minimum of fumbling, she looked up to see Derek watching her from the doorway. Clad only in sweatpants and nothing else, his hair was mussed and sticking out in all directions. The expression on his face was one that Casey had seen before but never quite been able to place: eyes dark and intense, for all that the curve of his lips was oddly gentle. 

Catching her gaze, Derek rubbed the back of his neck, and for a moment Casey almost _wanted_ him to bolt from the apartment before the situation could get any more horrendously awkward. Then his eyes widened as he noticed the food on the counter. “Are those chocolate chip pancakes?” he asked, then grabbed one before she could answer and stuffed it in his mouth. “These are amazing,” he said through a mouthful of half-chewed pancake. “Marry me, Casey.”

Casey rolled her eyes. “Don’t talk with your mouth full.” But even as she scolded him, she couldn’t keep the relief out of her voice.

* * *

Aside from some initial awkwardness, a few new rules about what displays of affection were and weren’t allowed in the living room, and some money transferring hands between Edwin and Lizzie which Casey had pretended not to notice, the family had been pretty cool about the whole Casey-and-Derek thing. Which was great! Casey was glad not to have to stumble through unconvincing lies about her relationship status or make up reasons why she and Derek had wanted to get an apartment together after graduation, grateful that her family seemed comfortable with their relationship.

It was just that...well, sometimes they were a little _too_ comfortable. A little _too_ willing to—just for example!—watch Casey and Derek bickering over the dinner table, amusement in their eyes as their heads swiveled back and forth like spectators at a tennis match.

“Casey,” Derek said, gesturing emphatically with a fork laden with mashed potatoes. “We are not getting a parrot for a pet. No way.”

“But they’re so smart!” Casey pleaded. “They can talk, and use tools.” She gave him her best puppy-dog eyes. “I’ll even let you teach it swear words.” 

“They live for like fifty years!” Derek shot back. “That is way too much of a commitment for a pet. Why don’t we start out with a goldfish?”

Casey set down her fork. “Oh, ‘commitment’! I should have known that was the problem! You’re afraid of commitment.”

“Oh my god,” said Derek. “Don’t turn this into a metaphor for our relationship. You know how much I sucked at English class.”

Casey folded her arms across her chest. “You were the one who brought up the c-word!”

“I—you know what?” Derek stood up from the table abruptly. “Hold that thought.” 

Casey watched his retreating back in confusion as he marched up the stairs. Derek _liked_ arguing with her; she couldn’t even remember the last time he’d just walked away in the middle of a debate. Glancing around the table, she was met with equally quizzical looks from the rest of her family.

“You know,” her mom said sympathetically, after a few moments of awkward silence. “Derek is right that parrots live for decades.” From upstairs, Casey heard the sound of a door being flung open.

“Goldfish can live for a long time, too!” Lizzie interjected. “They only die after a few years when people don’t take care of them properly.”

Edwin rubbed his chin thoughtfully. “I think you should get a pet tarantula.” 

Casey shuddered. “You know what?” she said, rising from her seat, “I think I’m going to go check on Derek.”

She only made it a few steps before footsteps echoed again from the direction of the stairs. “Hey, Casey,” Derek called. “Think fast!”

Years of living in the same house as him had honed Casey’s reflexes, and she spun to face him while throwing up her arms to cover her head. Derek was standing halfway up the steps, leaning over the banister and holding...a Pringles can? It was one of the small ones, the kind you got at the airport or something, where you were actually _supposed_ to be able to eat the whole thing in one sitting.

Derek lobbed the Pringles can at her, overhand. It arced through the air and, despite her best efforts to catch it, hit Casey on the head. She did manage to grab it before it fell to the ground, and was surprised by the metallic noise of something rattling inside. Curious, she pried off the lid.

And froze.

“You wanted some commitment, Casey?” Derek asked. She looked up at him, wide eyed, to see him descending the stairs and advancing towards her. 

Casey turned the Pringles can over, tipping the ring out into her hand along with a few greasy remnants of potato chip.

“Well,” Derek said as he reached her, "here’s some fucking commitment _._ ” He got down on one knee.

The rest of the house had gone silent. Or if they hadn’t, Casey couldn’t hear them over the buzzing in her ears.

“Casey McDonald,” Derek said. “Casey, Casey, _Casey._ You are the most frustrating person I have ever met in my entire life. By like, a lot.”

“Back atcha,” Casey responded reflexively.

“ _And_ ,” Derek continued, “I’ve been in love with you since I was fifteen years old. So. Marry me?”

Her whole face was hot. Her throat had tightened up and her eyes were swimming, while her heart threatened to pound its way out of her chest. “You—” she stammered. “You—that was a _terrible speech,_ Derek! And there are, there are crumbs! On the ring!”

Derek raised an eyebrow up at her. “Is that a no?”

“No!” Casey shrieked. “I mean, no, it’s not a no, _yes_ that it’s a—”

Derek rose to his feet. “I know what you mean,” he said, and kissed her.


End file.
